1. Field of the Invention
The present invention relates to an inductive head with an insulation stack whose height is reduced by a partial coverage zero throat height (ZTH) defining insulation layer and, more particularly, to an inductive head that has a ZTH defining insulation layer located entirely between an air bearing surface (ABS) of the head and one or more coil layers of an insulation stack, thereby permitting the insulation stack to be lowered.
2. Description of the Related Art
An inductive write head includes a coil layer embedded in first, second and third insulation layers (called "the insulation stack"), the insulation stack being located between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted through the coil layer produces a magnetic field in the pole pieces. The magnetic field fringes across the gap at the ABS for the purpose of writing information in tracks on moving media, such as in circular tracks on a rotating magnetic disk or longitudinal tracks on a moving magnetic tape.
The second pole piece layer has a pole tip portion which extends from the ABS to a flare point, and a yoke portion which extends from the flare point to the back gap. The flare point is where the second pole piece begins to widen (flare) to form the yoke. The placement of the flare point directly affects the magnitude of a magnetic field produced to write information on the recording medium. Since the magnitude of magnetic flux decays as it travels down the length of the narrow second pole tip, shortening the second pole tip will increase the magnitude of the flux reaching the recording media. Therefore, performance can be optimized by aggressively placing the flare point close to the ABS.
Another parameter important in the design of a write head is the location of the zero throat height (ZTH). The zero throat height is the location where the first and second pole pieces first separate from one another after the ABS. ZTH separation is imposed by an insulation layer, typically the first insulation layer in the insulation stack. Flux leakage between the first and second pole pieces is minimized by locating the ZTH as close as possible to the ABS.
Unfortunately, the aforementioned design parameters require a tradeoff in the fabrication of the second pole tip. The second pole tip should be well-defined in order to produce well-defined written tracks on the rotating disk. Poor definition of the second pole tip may result in overwriting of adjacent tracks. A well-defined second pole tip should have parallel planar side walls which are perpendicular to the ABS. This definition is difficult to achieve because the second pole tip is typically formed along with the yoke after the formation of the first insulation layer, the coil layer and the second and third insulation layers. Each insulation layer includes a hard-baked photoresist having a sloping front surface.
After construction, the first, second and third insulation layers present front sloping surfaces which face the ABS. The ZTH defining insulation layer rises from a plane normal to the ABS at an angle (apex angle) to the plane. After hard baking of the insulation layers and deposition of a metallic seedlayer, the sloping surfaces of the insulation layers exhibit a high optical reflectivity. When the second pole tip and yoke are constructed, a thick layer of photoresist is spun on top of the insulation layers and photo patterned to shape the second pole tip, using a conventional photolithography technique. In the photo-lithography step, ultraviolet light is directed vertically through slits in an opaque mask, exposing areas of the photoresist which are to be removed by a subsequent development step. One of the areas to be removed is the area where the second pole piece (pole tip and yoke) is to be formed by plating. Unfortunately, when the ultraviolet light strikes the sloping surfaces of the insulation layers in a flaring region of the second pole piece, it is reflected forward, toward the ABS, into photoresist areas at the sides of the second pole tip region. After development, the side walls of the photoresist extend outwardly from the intended ultraviolet pattern, causing the pole tip plated therein to be poorly formed. This is called "reflective notching". As stated hereinabove this can lead to overwriting of adjacent tracks on a rotating disk. It should be evident that, if the flare point is recessed far enough into the head, the effect of reflective notching would be reduced or eliminated since it would occur behind the sloping surfaces. However, this solution produces a long second pole tip which quickly reduces the magnitude of flux reaching the recording medium.
The high profile of the insulation stack causes another problem after the photoresist is spun on a wafer. When the photoresist is spun on a wafer, it is substantially planarized across the wafer. The thickness of the photoresist in the second pole tip region is higher than other regions of the head since the second pole tip is substantially lower on the wafer than the yoke portion of the second pole piece. During the light exposure step the light progressively scatters in the deep photoresist like light in a body of water, causing poor resolution during the light exposure step.
Although an insulation stack with a double coil is higher than an insulation stack with a single coil, a double coil head is desirable because two smaller diameter coils can produce the same flux density as a single coil, with less reluctance. Less reluctance permits a faster rise time of the signal which results in a faster data rate. There is a need for a double coil head wherein the height of the insulation stack can be reduced so as to improve the construction of a photoresist frame for plating the second pole tip.
Still another problem is that, if the ZTH defining layer of the insulation stack is not an insulation layer above the last coil, the ZTH will be altered by subsequent processing steps. A seedlayer, employed for plating the coil, is removed from all locations except under the coil by milling with an ion beam. The milling strikes all surfaces on a wafer where rows and columns of magnetic heads are constructed. If, for instance, the ZTH defining layer is the first insulation layer of the insulation stack, removal of the seed layer will also remove part of the ZTH defining insulation layer. This will cause a recession of the original location of the ZTH defining insulation layer, which alters the design of the head.